The present invention relates to a surging prediction device for a centrifugal compressor and to a refrigeration apparatus using the surging prediction device.
In the case where there takes place the surging in a centrifugal compressor, there has been conventionally provided an arrangement for detecting this surging and preventing the compressor from surging, for example, by lowering the discharge pressure thereof. The surging detection is effected, for example, by a device shown in a block diagram of FIG. 13 (refer to Japanese Patent Application Publication No. 58-15639). The device is composed of a pressure detector 301 for detecting an abrupt discharge pressure drop connected to the discharge line 302, a surging detection circuit 303 for detecting surging upon receiving an output of the pressure detector 301 when there is an abrupt pressure drop within a predetermined set time, and a surging preventing device 304 operating in response to a signal from the surging detection circuit 303 indicating the occurrence of surging.
However, in the conventional arrangement, since the surging detection is effected by the surging detection circuit 303 based on an abrupt drop of the discharge pressure detected by the pressure detector 301, the detection of surging can be made only after the surging has actually occurred, and it is impossible to control so as to prevent surging before the occurrence thereof. Further, since the discharge pressure varies in a capacity control type centrifugal compressor wherein the capacity control thereof is made possible, there is a problem that the arrangement is not applicable to a capacity control type centrifugal compressor. Furthermore, there is also a problem that the structure of the arrangement is complicated, resulting in a cost increase.
Therefore, there is provided another surging prevention arrangement which has, as shown in FIG. 14, a measuring means 311 for measuring an input power (current) to a motor 312 of a centrifugal compressor 313, and a comparator 314 for comparing an input power or current measured by the measuring means 311 with the reference power or current set based on a predetermined discharge pressure or predetermined volume and discharge pressure and for predicting surging in comparison of the input power (current) with the reference power (current) so as to operate a surging preventing valve 315 (refer to Japanese Utility Model Application Laid-open publication No. 63-31292). More specifically, the arrangement is such that the opening ratio of the suction vane in the centrifugal compressor 313 is detected through the input current to the motor 312, the surging prevention valve 315 which is provided on a bypass line 316 bypassing a pressure reducing valve (not shown) is opened based on the vane opening ratio so as to bypass hot gas through the bypass line 316, whereby the amount of work by the centrifugal compressor 313 is reduced and the apparent volume thereof is increased so as to prevent the surging without stopping the centrifugal compressor 313.
In the surging prevention arrangement, however, although the surging line is predicted through measurement of the input power (current) and the relationship between the input power and the discharge pressure, since the head wherein surging will take place is related not only with the discharge pressure but also with the ratio of the discharge pressure to the suction pressure, namely, the compression ratio, the error will increase when the suction pressure fluctuates. Furthermore, in the case where surging is predicted based on the volume and the discharge pressure, though the change in the volume approximates to the change in the input power (current), since the approximating relationship varies depending on the voltage fluctuations, there is an error developed, resulting in a problem that a correct detection of surging can not be made.
Furthermore, in the case where the adiabatic head (kcal/kg) rises in the centrifugal compressor 313 due to, for instance, the adhesion of scale in the piping of the refrigeration system or mixing of air into the refrigerant circulating the refrigeration system, the regulation of the suction vane opening ratio can not cope with the surging and before the surging prevention valve is operated, the centrifugal compressor 313 may reach the suerging area.
Therefore, in the actual operation , the following procedures are required to prevent the centrifugal compressor 313 from surging.
FIG. 15 shows an occurrence state of surging in a capacity control type centrifugal compressor by taking an adiabatic head (kcal/kg) in the ordinate and a volume (m.sup.3 /min) in the abscissa. When the centrifugal compressor 313 is set so that the surging prevention valve 315 is opened, for example, at the vane opening ratio of 40%, the adiabatic head may rise to reach a point (P) above the surging line (SL) due to adhesion of scale onto the refrigeration piping. Therefore, in the actual operation it is necessary to set the vane opening ratio wherein the surging prevention valve 315 is opened, for example, at the vane opening ratio of 60%, taking the head rise due to the scale adhesion into consideration.
For this reason, the lower limit of the suction vane opening ratio can not be lowered below 60%, resulting in a problem that the lower operation limit of the capacity control for the compressor 313 becomes higher.